1. Field of the Invention
This invention relates generally to techniques for testing soils, and particularly, to techniques for preparing the surface of a region of soil for further testing.
2. Background Art
It is often important to determine, for example, at least by estimate, the resistance of a soil to liquefaction, the degradation characteristics of a soil, the dynamic shear modulus of a soil at low levels of shear deformation, and the variation in the dynamic shear modulus of a soil with shear deformation. Liquefaction is the total loss of the stiffness and strength of a saturated soil caused by increased pore water pressure which can result from cyclic loading. Degradation is the reduction in stiffness also due to the buildup of pore water pressure caused by cyclic loading. Degradation may or may not lead to liquefaction depending upon the type and state of the soil. Generally, the shear modulus of a soil is a function of shearing deformation. For example, most soils show reduced stiffness with increasing deformation under monotonically increasing loading.
Commonly, these properties, as well as others, are necessary for analysis which predicts the response of a site or foundation structure system to dynamic loading caused by earthquakes, ocean waves or mechanical vibrations. Conventionally, these properties have been determined by conducting laboratory tests on samples recovered from a site or by in situ field tests.
Laboratory testing of soil samples suffers from a number of problems. Particularly, the acts of recovering a sample, transporting it to a laboratory, and preparing the sample for a test, can so disturb a sample from its original state as to render questionable any test results obtained therefrom. In addition, it is often difficult to reproduce the original field environment (state of stress) of the sample because it is often difficult and costly to define the environment and because typical laboratory test apparatus are limited in their ability to reproduce environmental conditions. Therefore, laboratory tests are subject to error due to their failure to precisely account for environmental considerations. Safely accounting for the affects of these disturbances and the inability to maintain or reproduce existing environmental conditions in the laboratory may lead to excessively costly structures.
There are a variety of devices and means that are used to collect data, such as that referenced above, from a given soil sample during in situ testing. For example, a closed ended probe may be (1) penetrated into the ground at a controlled slow rate, thus simulating static noncyclic loading, but at the same time introducing severe failure into the local soil, or (2) driven into the ground by violent impacts, thereby causing severe and immediate failure of the soil adjacent to the cylinder. Also, as disclosed in one embodiment of applicants' U.S. Pat. No. 4,594,899, an open ended cylindrical device, with an inner cylinder that is rotated by an impulse or by an oscillatory motion, can also be used to collect the above referenced data. However, irrespective of the devices used to derive the sample to be tested, the test results may be affected by the disturbance of the soil due to the initial drilling of the borehole. The influence of the disturbance of the soil due to initial drilling of the borehole could have a significant impact on any measured data obtained.
The accuracy and consistency of the results of soil testing can be improved through the use of the present method and apparatus for preparing the surface of the soil prior to testing. In particular, the present invention, by lessening the soil disturbance in the area adjacent the soil sample, reduces uncertainties present with prior data accumulation methods and devices.